The invention concerns a process to recover 4-phenoxybiphenyl from mixtures containing the compound and its isomers, 3-phenoxybiphenyl and 2-phenoxybiphenyl.
Phenoxybiphenyls are produced as byproducts from industrial processes that produce diphenyl ether. Diphenyl ether is produced by the caustic hydrolysis of chlorobenzene, as is generally taught by Smith, Caustic Hydrolysis of Chlorobenzene to Diphenyl Oxide, U.S. Pat. No. 4,092,364 (May 30, 1978), the teachings of which are incorporated herein by reference. A byproduct stream from the process contains a major amount of the 2-phenoxybiphenyl isomer, with smaller proportions of the 4-phenoxybiphenyl and 3-phenoxybiphenyl isomers, 2,6-diphenylphenol and a variety of other compounds.
Phenoxybiphenyls are commercially useful as monomer precursors in the production of polyetherketone resins. For example, Brugel, Copolyetherketones, U.S. Pat. No. 4,704,448 (Nov. 3, 1987), teaches the preparation of polyetherketones by contacting diphenyl ethers with diacid halides in a Friedel-Crafts synthesis. The 4-phenoxybiphenyl structure is useful in preparing acceptable monomers. See, Brugel at column 3, lines 23-24. It is preferable to use a highly pure form of the 4-phenoxybiphenyl isomer when preparing the monomers.
Distillation of the byproduct mixture to obtain a highly pure 4-phenoxybiphenyl product is not practical. The three isomers and 2,6-diphenylphenol have similar chemical structures, and thus, it is not surprising that the compounds have roughly equivalent boiling points. Due to the close boiling points, it is not practical to recover the 4-phenoxybiphenyl isomer in a highly pure form by use of a distillation column.
Similarly, persons skilled in the art would expect roughly equivalent solubilities for the isomers in various solvents, as is the case with available physical properties such as boiling points. Therefore, one would also anticipate difficulty in attempting to recrystallize 4-phenoxybiphenyl from a mixture of the three isomers.
It is known in the art that phenoxybiphenyl compounds can be purified by recrystallization from ethanol. See, Stoesser et al., Aryl Oxides, U.S. Pat. No. 2,095,619 (Oct. 12, 1937) at Examples 1-3. However, Stoesser et al. do not teach recovery of 4-phenoxybiphenyl by recrystallization from a mixture that includes its isomers. It is clear that Stoesser et al. merely purify the product of a reaction designed to produce one isomer.
Therefore, it is desirable to discover a process to economically and easily recover a highly pure 4-phenpxybiphenyl solid from mixtures containing this compound and its isomers.